History of Western Philosophy - Bertrand Russell [91]
We come next to criticisms of the doctrine of Heraclitus. This is first pushed to extremes, in accordance, we are told, with the practice of his disciples among the bright youths of Ephesus. A thing may change in two ways, by locomotion, and by a change of quality, and the doctrine of flux is held to state that everything is always changing in both respects.2 And not only is everything always undergoing some qualitative change, but everything is always changing all its qualities—so, we are told, clever people think at Ephesus. This has awkward consequences. We cannot say 'this is white', for if it was white when we began speaking it will have ceased to be white before we end our sentence. We cannot be right in saying we are seeing a thing, for seeing is perpetually changing into not-seeing.3 If everything is changing in every kind of way, seeing has no right to be called seeing rather than not-seeing, or perception to be called perception rather than not-perception. And when we say 'perception is knowledge', we might just as well say 'perception is not-knowledge.'
What the above argument amounts to is that, whatever else may be in perpetual flux, the meanings of words must be fixed, at least for a time, since otherwise no assertion is definite, and no assertion is true rather than false. There must be something more or less constant, if discourse and knowledge are
to be possible. This, I think, should be admitted. But a great deal of flux is compatible with this admission.
There is, at this point, a refusal to discuss Parmenides, on the ground that he is too great and grand. He is a 'reverend and awful figure'. 'There was a sort of depth in him that was altogether noble.' He is 'one being whom I respect above all'. In these remarks Plato shows his love for a static universe, and his dislike of the Heraclitean flux which he has been admitting for the sake of argument. But after this expression of reverence he abstains from developing the Parmenidean alternative to Heraclitus.
We now reach Plato's final argument against the identification of knowledge with perception. He begins by pointing out that we perceive through eyes and ears, rather than with them, and he goes on to point out that some of our knowledge is not connected with any sense-organ. We can know, for instance, that sounds and colours are unlike, though no organ of sense can perceive both. There is no special organ for 'existence and non-existence, likeness and unlikeness, sameness and differences, and also unity and numbers in general'. The same applies to honourable and dishonourable, and good and bad. 'The mind contemplates some things through its own instrumentality, others through the bodily faculties.' We perceive hard and soft through touch, but it is the mind that judges that they exist and that they are contraries. Only the mind can reach existence, and we cannot reach truth if we do not reach existence. It follows that we cannot know things through the senses alone, since through the senses alone we cannot know that things exist. Therefore knowledge consists in reflection, not in impressions, and perception is not knowledge, because it 'has no part in apprehending truth, since it has none in apprehending existence'.
To disentangle what can be accepted from what must be rejected in this argument against the identification of knowledge with perception is by no means easy. There are three inter-connected theses that Plato discusses, namely:
(1) Knowledge is perception;
(2) Man is the measure of all things;
(3) Everything is in a state of flux.
(1) The first of these, with which alone the argument is primarily concerned, is hardly discussed on its own account except in the final passage with which we have just been concerned. Here it is argued that comparison, knowledge of existence, and understanding of number, are essential to knowledge, but cannot be included in perception since they are not effected through any sense-organ. The things to be said about these are different.